The invention relates to a common rail injector for injecting fuel in a common rail injection system of an internal combustion engine, which system has an injector housing with a fuel inlet that is in communication with a central high-pressure fuel reservoir outside the injector housing and with a pressure chamber inside the injector housing, from which fuel subjected to high pressure is injected as a function of the position of a control valve that assures that a nozzle needle movable back and forth and received in a longitudinal bore of the injector axially counter to the prestressing force of a nozzle spring that is received in a nozzle spring chamber, lifts from a seat when the pressure in the pressure chamber is greater than the pressure in a control chamber that communicates with the fuel inlet via an inlet throttle and with a relief chamber via a fuel outlet.
In common rail injection systems, a high-pressure pump pumps the fuel into the central high-pressure fuel reservoir, which is called a common rail. From the high-pressure fuel reservoir, high-pressure lines lead to the individual injectors, which are assigned to the engine cylinders. The injectors are triggered individually by the engine electronics. The rail pressure prevails in the pressure chamber and at the control valve. When the control valve opens, fuel subjected to high pressure reaches the combustion chamber, past the nozzle needle that is lifted counter to the prestressing force of the nozzle spring.
In conventional injectors, of the kind known for instance from German Patent Disclosure DE 197 24 637 A1, relatively long nozzle needles are used. In operation, because of the high pressures and the rapid load changes, very strong forces act on the nozzle needle. These forces cause the nozzle needle to be stretched and compressed in the longitudinal direction. This in turn means that the nozzle needle stroke varies as a function of the forces acting on the nozzle needle.
The object of the invention is to furnish a common rail injection system with a small structural volume that is simple in design and can be produced economically. In particular, even at a high nozzle needle speed, good closing performance should be assured.
In a common rail injector for injecting fuel in a common rail injection system of an internal combustion engine, which system has an injector housing with a fuel inlet that is in communication with a central high-pressure fuel reservoir outside the injector housing and with a pressure chamber inside the injector housing, from which fuel subjected to high pressure is injected as a function of the position of a control valve that assures that a nozzle needle movable back and forth and received in a longitudinal bore of the injector axially counter to the prestressing force of a nozzle spring that is received in a nozzle spring chamber, lifts from a seat when the pressure in the pressure chamber is greater than the pressure in a control chamber that communicates with the fuel inlet via an inlet throttle and with a relief chamber via a fuel outlet, this object is attained in that the control chamber is integrated with the end of the nozzle needle remote from the combustion chamber. This furnishes a compact common rail injector which assures rapid closure of the nozzle needle. The control chamber can be made smaller than in conventional injectors, which makes for a rapid response performance by the injector. Embodying the injector according to the invention makes rail pressures of up to 1800 bar possible.
One particular type of embodiment of the invention is characterized in that in the end, remote from the combustion chamber, of the nozzle needle a substantially cylindrical recess is provided, in which an outer circumferential portion of a bush is axially displaceably received with a sealing effect, the end face, remote from the combustion chamber, of which bush is pressed by the prestressing force of the nozzle spring against the injector housing, and the interior of which bush communicates with the fuel outlet. The bush offers the advantage that the control chamber and the nozzle spring chamber can be combined on the end, remote from the combustion chamber, of the nozzle needle without the volume of the control chamber being dependent on the installation space for the nozzle spring. It is therefore possible to build in a nozzle spring with a high spring stiffness, which assures good closure of the nozzle needle. As a result, the duration and instant of injection can be defined exactly.
A further particular type of embodiment of the invention is characterized in that on the end of the bush remote from the combustion chamber, a collar is embodied, which forms an abutment for the nozzle spring that is prestressed counter to the end, remote from the combustion chamber, of the nozzle needle. In the context of the present invention, the nozzle spring has a dual function. First, the closing motion of the nozzle needle is effected by the prestressing force of the nozzle spring, and second, the control chamber volume is defined by the prestressing force of the nozzle spring in conjunction with the pressure in the control chamber.
Another particular type of embodiment of the invention is characterized in that on the face end of the bush remote from the combustion chamber, which is in contact with the injector housing, a biting edge is embodied. As a result, the control chamber is separated from the nozzle spring chamber surrounding the bush.
Another particular type of embodiment of the invention is characterized in that the fuel inlet communicates with the pressure chamber via the nozzle spring chamber, and that the nozzle needle is guided between the nozzle spring chamber and the pressure chamber. This offers the advantage that the nozzle needle guide no longer has any sealing function. This makes the demands in terms of quality of the guide less stringent, leading to economies in production. Since the same pressure prevails on both sides of the guide, guide leakage no longer occurs.
A further particular type of embodiment of the invention is characterized in that at least one flat face, past which fuel from the nozzle spring chamber can reach the pressure chamber, is embodied on the nozzle needle between the nozzle spring chamber and the pressure chamber. This type of embodiment offers advantages especially with regard to the high-pressure strength.
Another particular type of embodiment of the invention is characterized in that the inlet throttle is integrated with the nozzle needle or the bush. The inlet throttle serves to prevent pressure surges in operation.
A further particular type of embodiment of the invention is characterized in that the nozzle needle stroke is defined by the spacing between the nozzle needle and the bush. This purely mechanical nozzle needle stroke end stop offers the advantage that the nozzle needle stroke is exactly replicable. As a result, the course of injection can be shaped reliably. So-called hydraulic sticking is avoided.
A further particular type of embodiment of the invention is characterized in that on the end of the bush remote from the combustion chamber, an auxiliary control chamber is embodied, which communicates via an inlet throttle with the nozzle spring chamber and via an auxiliary throttle with the cylindrical recess in the nozzle needle. In this type of embodiment, during the closing motion of the nozzle needle, the bush can be lifted from its seat on the injector housing. As a result, the auxiliary control chamber and the cylindrical recess that forms the actual control chamber can be more quickly filled with fuel that is at high pressure. This further speeds up the closing motion of the nozzle needle.